Small-scale microelectrodes are used in a number of applications. One promising field of application of microelectrodes is for neural prosthetic medical devices, which are typically used to stimulate nerve cells to overcome pre-existing medical conditions. Examples of such medical conditions include diseases producing damage to the retina such as uveitis, retinitis pigmentosa, macular degeneration, diabetic retinopathy and glaucoma. Metallic microelectrodes have been used for neural prosthetic medical devices. Platinum and platinum-based alloys have proven useful for such microelectrodes because of the high degree of bio-compatibility afforded.
Platinum and iridium, and the platinum group metals in general, possess the unique qualities of having multiple oxidation states, conducting oxides, low electrochemical impedance, and enhanced biocompatibility in electrical stimulating and sensing applications over other conducting metal materials. These qualities position both platinum and iridium as prime choices for electrode composition in applications such as fuel cell electrodes, hydrolysis reactions electrodes, and implantable electrodes for sensing and stimulating. Individually, platinum and iridium possess different mechanical properties. Platinum is known to be ductile and malleable, while iridium is known to be stiff and brittle. The two are often alloyed to create alloys with improved mechanical properties over the individual constituents. The exact ratio of the two is adjusted so as to match the mechanical property requirements for the particular application, e.g. more platinum where a more ductile electrode is preferred and more iridium were stiffness is desired. A common quality of both materials is high melting temperature, platinum (1772° C.) and iridium (2410° C.). This makes processing and handling of both metals a challenge, particularly in the manufacturing of small components. A variety of thin film processing techniques have been used to create components and features of this size, e.g., electron beam evaporation and magnetron sputtering in concert with photolithography and other semi-conductor processing techniques; however, these processes are performed under high-vacuum which is not time-efficient or cost-effective. Additionally, these are thin film coating processes that coat series of mono-atomic layers of metal throughout the deposition chamber thus being source-material inefficient.
Another quality of both metals worth noting is their low electrochemical impedance. This is attributable to the multiple oxidation states of both elements, as well as, the electrical conductivity of their oxides that allow for easier electron transfer between the metal and a surrounding electrolyte solution. Iridium possesses more oxidation states and its oxide possesses a lower electrical resistance therefore it shows lower electrochemical impedance over platinum and platinum oxide.
Some previous techniques for electroplating platinum and/or platinum iridium have been reported. Such techniques, however, have typically utilized corrosive and/or toxic solutions. Moreover, such techniques have produced microelectrodes having less than ideal mechanical properties and/or electrical properties.